linux-kernel - RE: [RFC PATCH 5/5] locking/rwsem: Remove reader optimistic spinning

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Message-ID: <45f36a7545c84b62b49aaff95f28a49a@AcuMS.aculab.com>
Date:   Sat, 21 Nov 2020 11:50:22 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Davidlohr Bueso' <dave@...olabs.net>
CC:     'Waiman Long' <longman@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Phil Auld <pauld@...hat.com>
Subject: RE: [RFC PATCH 5/5] locking/rwsem: Remove reader optimistic spinning

From: Davidlohr Bueso
> Sent: 20 November 2020 21:38
> 
> On Fri, 20 Nov 2020, David Laight wrote:
> >I got massive performance improvements from changing a driver
> >we have to use mutex instead of the old semaphores (the driver
> >was written a long time ago).
> >
> >While these weren't 'rw' the same issue will apply.
> >
> >The problem was that the semaphore/mutex was typically only held over
> >a few instructions (eg to add an item to a list).
> >But with semaphore if you got contention the process always slept.
> >OTOH mutex spin 'for a while' before sleeping so the code rarely slept.
> 
> The caveat here is if you are using trylock/unlock from irq, which
> is the only reason why regular semaphores are still around today. If
> not, indeed a mutex is better.

Unless you want to timeout the lock request.

Timeouts are particularly useful in code paths that might
run after an 'oops' or other deadlock.
Typically for reporting status information.
You get to choose whether to error the status request or
carry on knowing that the data is unlikely to change.

	David

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